Duplicate and scale edges but keep in the same mesh
There may be a snappier way, but I would:


CtrlB bevel the outer curve of the wheel-arch, Segments 2, Profile 1
CtrlX dissolve the surplus loop on the inside of the arch
Select the new edges, and GG with C Loop Slide to adjust the depth of the new loop's offset.. 




..depending on your previous use of the Loop Slide, you may have to hit E to get the sliding loop to be parallel to one side or the other, and maybe F to flip which side.